Plastic materials are widely used as substitutes for glass in many applications, and particularly for optical elements such as ophthalmic lenses. Plastic materials have a number of advantages over glass, such as lighter weight, ease of handling and ease of formation of articles. However, plastic materials rarely have all of the physical and/or optical properties required for a particular application. Most notably, plastic materials are soft and scratch quite readily in comparison to glass.
It has become customary to coat plastic optical elements with coatings to provide them with the desired physical or optical properties. For example, plastic optical elements such as ophthalmic lenses are generally coated with abrasion resistant “hard coats” to reduce abrasion on the surface of the element. Furthermore, optical elements such as ophthalmic lenses often include an anti-reflection (“AR”) coating to improve transmittance of visible light.
Whilst coatings, such as hard coats and AR coats, can be advantageous in terms of improving the optical or physical properties of an optical element, they can also lead to problems. For example, there can be problems with adhesion of hard coating layers to an optical element, or to other coating layers, and AR coatings are brittle and they crack relatively easily. These problems can reduce the product life of optical elements containing these coatings.
The problem of cracking of AR coated optical elements has become more prevalent in recent times in the ophthalmic lens field because of a trend towards thinner lenses. For aesthetic reasons, lenses having a centre thickness of about 1 mm are sought after. Prior to these thinner lenses being made available, lenses having a centre thickness of 1.5 mm and 2.0 mm were more common. The trend toward thinner lenses has emphasized problems with cracking of the coatings. Clearly, there is a need for optical elements, such as ophthalmic lenses, having an AR coating and also having improved impact resistance.
The prior art contains a number of suggestions for providing primer layers for improving the impact resistance of optical elements and the adhesion of coating layers on the optical element. U.S. Pat. No. 6,051,310 to Cano et al. describes an ophthalmic lens having an abrasion resistant coating, a layer of impact resistant primer, and an inorganic AR coating. The primer layer, which is inserted between the abrasion resistant coating and the AR coating, is formed using an epoxysilane based polymer. U.S. Pat. No. 6,551,710 to the present applicants also describes a primer composition that can be used to form impact resistant primer layers on ophthalmic lenses having hard coats and AR coats. The primer layer is formed using a thiolene based polymer. Published United States patent application 2003/0118833 to Essilor describes the formation of a primer layer from a non-hydrophilic acrylate monomer and an epoxy monomer.
Some of the prior art primer layers referred to above may lead to an improvement in the impact resistance of an optical element. However, coatings of this type can lead to further problems resulting from poor adhesion of the optical element substrate and/or the coatings to the primer layer. Poor adhesion subsequently reduces the product life of the optical element. For example, we have found that it is necessary to pre-treat the surface of an ophthalmic lens with corona discharge prior to depositing the primer layer described in U.S. Pat. No. 6,551,710 in order to enhance adhesion of the primer layer to the lens. This additional step adds to the final cost of production of a lens.
It is an aim of the present invention to provide a coated optical element having a primer layer that improves impact resistance and adhesion and preferably overcomes or alleviates a problem with the prior art, or at the very least provides manufacturers with a viable alternative to existing technology.
A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that that document or matter was known or that the information it contains was part of the common general knowledge.